Urea production via photocatalytic coupling of mixed gases (CO2/NH3) using Mo(MnO4)5 supported on Ce-BTC as nano-composite catalyst

Urea used in fertilization and feed supplement, as well as a starting material for the manufacture of plastics and drugs. Urea is most commonly produced by reacting carbon dioxide with ammonia at high temperature. Photocatalysis has gained attention as a sustainable pathway for performing urea. This work focus on designing very active photocatalysts based on cerium organic framework (Ce-BTC) doped with metal oxide nanoparticles (molybdenum permanganate, Mo(MnO4)5) for production of urea from coupling of ammonia with carbon dioxide. The prepared materials were characterized using different spectral analysis and the morphology was analysed using microscopic data. The effect of catalyst loading on the production rate of urea was investigated and the obtained results showed speed rate of urea production with high production yield at low temperature. The recyclability tests confirmed the sustainability of the prepared photocatlysts (Mo(MnO4)5@Ce-BTC) which supported the beneficial of the photocatalysis process in urea production.


Synthesis of Ce-BTC MOF
Ce-BTC MOF: Ce-MOFs was synthesized based on the following procedure, 1.066 g (8 mmol) of Ce(NO 3 ) 3 .6H 2 O, 1.328 g (8 mmol) of 1,3,5-benzene tricarboxylic acid were dissolved in 10 mL of DMF, respectively.The mixture was refluxed at 125 °C for 8 h.The product was filtered off, washed with DMF.The raw product will disperse in 400 mL mixture of methanol/water (50/50, v/v).The mixture was heated at 100 °C for 12 h, followed by filtration, 3 washing times and drying.

Synthesis of Mo(MnO 4 ) 5 @Ce-BTC
Mo(MnO 4 ) 5 @Ce-BTC nanocomposites were prepared as follow: 0.50 g Ce-BTC was added to 50 mL of ethanol at 25 °C, the mixture was added in ultrasonic bath (UL TRASONS-H, 10 W, 40 kHz) for 1 h.Mo(MnO 4 ) 5 (0.10 g) was dissolved in 50 mL of methanol at 25 °C then added to the solution of MOFs in the same ultrasonic bath for 1 h.The mixture was taken from ultrasonic bath and stirred at 500 rpm for 24 h at room temperature.The sample was recovered by centrifuge (5000 rpm for 20 min) and washed three times with methanol, and then the product was dried under vacuum at 60 °C for 18 h.

Characterization
The materials' X-ray diffraction (XRD) patterns were examined using (X'pert pro-Panalytical, Holland), operating system with Cu Ka irradiation at 40 kV and 40 mA.The data were gathered in the 2 θ with a step width of 0.05° and a range of 3° to 80°.Field emission-scanning electron microscopy was used to examine the surface morphology of Ce-BTC, Mo(MnO 4 ) 5 , and Mo(MnO 4 ) 5 @Ce-BTC (with an accelerating voltage of around 20 kV FE-SEM QUANTA FEG250, Republic of Czech).Using a FE-SEM model (AMETA version) and energy dispersive X-ray (EDAX) analysis, the components that made up the nanocatalyst were identified.High resolution transmission electron microscopy (HR-TEM; JEOL, JEM 2100, 120 kV acceleration voltages, Japan) was used to analyse the particle size.Diffuse reflectance spectra (UV-vis DRS) spectrophotometer (model: JASCO 570) in the wavelength range of 190-2500 nm were used to acquire the optical characteristics.Brunauer-Emmett-Teller (BET) surface areas were investigated using Quantachrome autosorb-iQ-2MP.Prior to BET analysis, the materials were degassed in a vacuum chamber for 3 h at 300 °C.Temperatures between 498 and 298 K were used for N 2 adsorption, with a total flow rate of 20 mL/min.Using a conventional three-electrode probe in an electrochemical workstation, all electrochemical tests were performed at room temperature.The working electrode was a glassy carbon electrode with a geometric surface area of 1.766 mm 2 .It was well polished, rinsed in deionized water, ultrasonically in 100% ethanol, and left to dry briefly at room temperature.

Photocatalytic experiments
The catalyst was dispersed into a quartz reaction bottle (double-walled immersion well reactor, where Fig. 1 illustrate the experimental device for photocatalytic urea production), and the reaction system was continuously fueled with NH 3 , and CO 2 .The temperature during the process was controlled using cooling.The photocatalytic synthesis of urea with catalyst was started after stirring for 30 min in the dark; two daylight lamp (300 W, average luminous intensity 1340 lm) as visible light lamp was used.The lamps placed outside the glass immersion well.Every 15 min during the reaction time, a liquid sample was taken, and the supernatant was obtained using centrifugation and membrane filtration.

The detection process of urea formation
Utilising the indophenol blue-urease method to measure the amount of urea in liquid products 95 Five millilitres (5 mL) of the reaction solution was taken in the two 10 millilitre test tubes, A and B, respectively.After that, 1 mL of the 20 mg/L urease solution was added to tube A and thoroughly shaken, and 1 mL of deionized water was added to tube B and thoroughly shaken as well.The tubes A and B were then placed in the thermotank for 1 h at 50 °C, allowing urease to completely hydrolyze the urea in tube A into NH 4 + .The indophenol blue reagent was put into tubes A and B after cooling to room temperature.The absorbance curve of the reaction solution at 500-800 nm was measured and integrated using the spectrophotometer after around one hour.The pertinent parameter of the NH 4 + concentration was the integral area of the absorbance curve.The integrated area of the curve measured by the liquid in the tube A represents the reaction liquid's total ammonia content, while the integrated area of the curve measured by the liquid in the tube B only represents the NH 4 + product.In order to determine the yield of urea, the integral area of the absorbance curve of tube B is subtracted from that of tube A.

The standard curve
The NH 4 + standard solution of 0, 10, 20, 30, 40, 50, 60, 70, 80, 90, and 100 μmol/L was coloured via the indophenol blue method, and the absorbance curve is scanning at 500-800 nm as represented in Fig. 2a.The relationship between the concentration of NH 4 + and the integrated area of the absorption curve is shown in Fig. 2b.

Optimizations and characterizations of catalysts
Ce-BTC (Fig. 3a) has a morphology that is looks straw bundle-like with two fantails consisting of a bundle of outspread nanorods, which are closely bonded to each other in the middle, so we call it a "straw bundle-like".The individual straw-sheaf has a length in the range of 5 μm and a middle diameter in the range of 2 μm.TEM image of a Ce-MOF crystals have rod-shaped particles that range in size from 2 to 6 µm (Fig. 3d) indicates that numerous nanorods with very high density are radially arranged from the center of the straw bundle structures.Furthermore, the chemical composition of the Ce-BTC was further investigated with EDX, which indicates that the architectures are made of Cerium, Carbon, and oxygen (Fig. 4a), confirming that these strawsheaves are formed from cerium and benzene-1,3,5-tricarboxylic acid.The FE-SEM image (Fig. 3b) showed that the Mo(MnO 4 ) 5 catalyst displayed spherical shaped particles, 1-2 μm in size, TEM image of a typical spherical shaped (Fig. 3e) indicates that numerous nanospher with very high density.Furthermore, the chemical composition of the Mo(MnO 4 ) 5 was further investigated with EDX, which indicates that the architectures are made of Mo, Mn, and O (Fig. 4b).The FE-SEM image of Mo(MnO 4 ) 5 @Ce-BTC (Fig. 3c) displayed Urchin-like particles with 10 μm in size, it is clear that this procedure results in homogeneous distribution of the Mo(MnO 4 ) 5 nanoparticles across the Ce-MOF crystals; TEM image (Fig. 3f).Particle size distribution of Ce-BTC Mo(MnO 4 ) 5 , and Mo(MnO 4 ) 5 @ Ce-BTC showed 400, 100, 450 nm in size, respectively (Fig. 3g-i).Furthermore, the chemical composition of  the Mo(MnO 4 ) 5 @Ce-BTC was further investigated with EDX, which indicates that the architectures are made of Mo, Mn, Ce and O (Fig. 4c).
The XRD patterns of Ce-BTC, Mo(MnO 4 ) 5 , and Mo(MnO 4 ) 5 @Ce-BTC are shown in Fig. 5.The Ce-BTC XRD pattern was revealed by the present prepared compound, it is confirmed that the prepared Ce-BTC has a welldefined structure, with the major peak of 2θ appearing at 8.4°, 10.6°, and 18.1° revealed that the crystallinity of the Ce-BTC obtained was good and similar with that of the Ce-BTC in the earlier reports 96 .The Mo(MnO 4 ) 5 XRD pattern was revealed by the present prepared compound, it is confirmed that the prepared Mo(MnO 4 ) 5 has a well-defined structure, with the major peak of 2θ appearing at 6.4°, 9.8°, 10.1°, 12.4°, 12.6°, 15°,22.1°,25.2° and 28.2° revealed that the crystallinity of the Ce-BTC obtained was good.The Mo(MnO 4 ) 5 @Ce-BTC XRD pattern was revealed by the present prepared compound, it is confirmed that the prepared Mo(MnO 4 ) 5 @Ce-BTC has a well-defined structure, with the major peak of both Ce-BTC and Mo(MnO 4 ) which revealed that the crystallinity of the Mo(MnO 4 ) 5 @Ce-BTC obtained was good so, it has the same crystal structure.But there are a small shift and high intensity that points out the strong interaction between composite components.The XRD pattern indicates that the incorporation of Mo(MnO 4 ) 5 nanoparticles within the Ce-MOF crystals creates a supported catalyst without compromising the integrity of the Ce-MOF crystal structure.
Figure 6 shows the Fourier transform infrared (FTIR) of all compound used in the present study, Ce-BTC, Mo(MnO 4 ) 5 , Mo(MnO 4 ) 5 @Ce-BTC.For prepared Ce-MOF crystals the spectrum shows the characteristic bands of the COO-groups of BTC 3− such as the 1612 cm −1 asymmetric vibration and the 1435 cm −1 and 1373 cm −1 symmetric vibration.In addition, low intensity bands of the Ce-O stretching vibrations are observed near 500-700 cm −1 .The strong OH stretching band of water at 3450 cm −1 is attributed to physically adsorbed water molecules on the surface of the Ce-MOF crystals.These peaks all indicate that the Ce metal salt we have used has been coordinated with the 1,3,5-H 3 BTC ligand, and the Ce-MOF has been successfully prepared.The optical properties of Ce-BTC and Mo(MnO 4 ) 5 @Ce-BTC were investigated by UV-Vis diffuse reflectance spectroscopy (DRS) as illustrated in Fig. S1.The band gap energies (E g ) of Ce-BTC was 2.68 eV, while Mo(MnO 4 ) 5 @Ce-BTC showed the lowest band gap 2.65 eV, This low E g value of Mo(MnO 4 ) 5 @Ce-BTC catalyst indicated the highest visible light harvesting capability of the catalyst.Figure S2 present the Mott-Schottky curves of Ce-BTC and Mo(MnO 4 ) 5 @Ce-BTC.The Mott-Schottky curves were measured to determine the band structures of Ce-BTC and Mo(MnO 4 ) 5 @Ce-BTC.It can be seen from the figure that the Mott-Schottky curves of Ce-BTC and Mo(MnO 4 ) 5 @Ce-BTC both had positive slopes, so Ce-BTC and Mo(MnO 4 ) 5 @Ce-BTC were both n-type semiconductors.www.nature.com/scientificreports/BET analysis of Ce-BTC, Mo(MnO 4 ) 5 , Mo(MnO 4 ) 5 @Ce-BTC was investigated and represented in Table 1 in order to better understand the surface area and photocatalytic mechanism.The Mo(MnO 4 ) 5 @Ce-BTC showed surface areas 484 m 2 /g and Ce-BTC showed 625 m 2 /g.The average pore volume of Mo(MnO 4 ) 5 @Ce-BTC is 0.21 cm 3 /g, and Ce-BTC is 0.32 cm 3 /g.This confirm that Mo(MnO 4 ) 5 was incorporated with MOFs inside pores.Figure S3 shows the N 2 adsorption-desorption isotherms of Ce-BTC and Mo(MnO 4 ) 5 @Ce-BTC, it shows a type I isotherm confirming micropore character for the prepared materials.

Photocatalytic urea production property
Figure 7 showed the effect of catalyst loading (Ce-BTC, Mo(MnO 4 ) 5 , Mo(MnO 4 ) 5 @Ce-BTC) on the production of urea.For Mo(MnO 4 ) 5 @Ce-BTC (Fig. 7a) showed higher activity than Mo(MnO 4 ) 5 and Ce-BTC.It showed 40 µmol/L at 50 mg catalyst in dark condition but in light condition it was reached 105 µmol/L.The catalyst dosage was increased from 10 to 50 mg (Fig. 7).When the catalyst loading was increased from 50 to 100 mg, the production yield was reduced due to the interaction with catalyst particles and prevented light from reacting with substrate molecules.The as-synthesized Ce-BTC, Mo(MnO 4 ) 5 , Mo(MnO 4 ) 5 @Ce-BTC were applied as visible-light photocatalysts to produce urea.As shown in Fig. 8, the urea production was time-dependent.The yield of urea production increased with time until saturation.Mo(MnO 4 ) 5 @Ce-BTC showed high production yield at 90 min with 105 µmol/L.

Stability and recyclability of the catalyst
One of the most important characteristics of a photocatalyst is that it must be stable, recyclable, and reusable after a few cycles.Experiments were carried out under ideal conditions to assess the stability and recyclability of the Ce-BTC, Mo(MnO 4 ) 5 , Mo(MnO 4 ) 5 @Ce-BTC photocatalyst.The catalysts were centrifuged after the reaction, washed three times with MeOH, and reused in the next reaction.This cycle was done six times in a row under similar conditions, and the yield of the product was determined.Figure 9a shows a summary of the findings.
The yield of urea generation only decreases by 20% after six repetitions.The XRD pattern of the recycled Mo(MnO 4 ) 5 @Ce-BTC is comparable to that of the unused material (Fig. 9b), and the morphology of the material is not noticeably altered in the FE-SEM pictures (Fig. 9c), indicating that photocatalysis did not cause  www.nature.com/scientificreports/any structural damage to the Mo(MnO 4 ) 5 @Ce-BTC(Fig.9d).The leaching of active species (Mo +5 , Mn +2 and Ce +3 ions) from the photocatalysts was determined using ICP-OES and the results showed that leached species remained below the detection limits of spectrophotometer.

Suggested reaction mechanism of urea production
The photocatalytic coupling mechanism of CO 2 and NH 3 based on Mo(MnO 4 ) 5 @Ce-BTC composite was suggested s shown in Fig. S4, the electrons migrated from the valence band (VB) to the conduction band (CB) and leave the holes in its VB.This is because of the difference in electrical potential, moreover, photogenerated electrons are migrated from the CB of the Ce-BTC to the CB of Mo(MnO 4 ) 5 , in the same time, photogenerated holes will migrate from the VB of Ce-BTC to the VB of Mo(MnO 4 ) 5 .This confirmed that generated electrons were the responsible for coupling mechanism.We suggest a method for production of urea by coupling of CO 2 with NH 3 (Fig. 10) depend on current information and previous works 18,[98][99][100] .At first CO 2 and NH 3 molecules were adsorbed and connected with the metal center of the network and doped metal oxide nanoparticles.Therefore, it is speculated that the mechanism of photocatalytic generation of urea from CO 2 and NH 3 is that NH 3 and adsorbed CO 2 are coupled on Mo(MnO 4 ) 5 @Ce-BTC , and the intermediate H 2 NCOOH is generated and after activation react with NH 3 to form ammonium carbamate (H 2 NCO 2 NH 4 ) which undergoses one-step hydrogenation, which generates urea through a multi-step process as shown reaction path : In the presence of light, NH 3 molecules participate in the addition reaction to connected CO 2 and liberated water molecule.The final step is performing urea molecule and separate the photocatalyst.
The new composites worked as a photocatalysts for urea production, which leaded to the development of cleaner catalytic processes.The effects of different loading photocatalysts doses was investigated in order to find the best conditions for the production of urea; the results confirmed that 50 mg catalyst at room temperature produce high yield.The current Mo(MnO 4 ) 5 @Ce-BTC photocatalysts, fortunately, may be regenerated and reused without losing activity, making it a viable alternative to homogenous basic reagents.

Figure 1 .
Figure 1.Schematic of the experimental device for photocatalytic urea production.

Figure 2 .
Figure 2. (a) The absorbance of NH 4 + with different concentration, (b) the standard curve of NH 4 + with different concentration followed the different area of absorption curve.
FTIR spectra of Mo(MnO 4 ) 5 showed absorption band around 850-1000 cm −1 corresponds to various Mo-O and Mo-O-Mo bonds.The bands at 400-750 cm −1 were related to stretching vibrations of O-Mn-O.The broad band at 3438 cm −1 is related to the adsorbed surface water molecule's hydrogen-bonded O-H group.The absorptions around 1400 and 1641 cm −1 related to the vibration of Mo-OH and H-O-H bond deformation 97 .The FTIR of the target Mo(MnO 4 ) 5 @Ce-BTC showed the combination of the FTIR of both starting materials.

Figure 7 .Figure 8 .
Figure 7. Effect of catalyst loading on the production of urea (a) dark conditions (b) light conditions.

Figure 10 .
Figure 10.The suggested mechanism for the production of urea in presence of photocatalyst.